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BELFAST, Northern Ireland — Police battled Irish nationalists for control of a Belfast road Monday as a day dominated by peaceful Protestant parades across Northern Ireland turned violent when night fell.

Riot police in helmets and body armor dragged kicking, flailing protesters from the pavement of Crumlin Road even as other protesters packed into side streets pelted police with rocks, bricks and Molotov cocktails.

Many of the approximately 100 road-blocking protesters wore white T-shirts bearing the message "PEACEFUL PROTEST," while the rioters nearby wore balaclava masks, hoods or scarves to conceal their faces. Police deployed a massive mobile water cannon to blast the rioters while a helicopter overhead monitored the mob.

An Associated Press reporter saw one teenage rioter holding a brick get knocked off his feet as he prepared to throw it. Blood streamed from his face as he scrambled from the pavement. Other rioters stood further back and threw empty beer bottles blindly over rooftops into police lines.

The violence — beside a traditional Irish Republican Army power base called Ardoyne — underscores how socially divided Northern Ireland remains despite nearly two decades of peacemaking that has delivered paramilitary cease-fires and a fragile Catholic-Protestant government.

It followed daylong parades across Northern Ireland by the Orange Order, a British Protestant brotherhood that each July 12 celebrates its side's 17th-century military triumphs over Irish Catholics. For the past decade, Ardoyne Catholics have protested — and often attacked — the small Orange parade that passes near the district.

Police said they had no doubt that officers and protesters were suffering multiple injuries in the latest conflict, but did not expect to have accurate casualty numbers until Tuesday. They said a policewoman suffered a suspected fractured skull after being struck in the head in Ardoyne, and paramedics' efforts to evacuate her were delayed by the rioters' unrelenting barrage.

The violence spread to other working-class parts of Belfast where rival Protestant and Catholic communities live cheek by jowl.

Police reported suffering salvoes of stones, bottles and Molotov cocktails near the Ormeau Bridge in south Belfast, which also was barricaded with burning trash cans.

The latest trouble comes on top of rioting in two other Catholic parts of Belfast early Monday. Police said 27 of their officers were hurt during those street battles, including three who suffered pellet wounds from a shotgun blast.

Politicians accused Irish Republican Army dissidents opposed to Northern Ireland's peace process of directing the riots and orchestrating a series of other attacks and threats across the British territory.

"Society wants to move forward, and the organized actions of the past 36 hours are doing nothing to reinforce the peace that the people of Ireland voted for," said Belfast mayor Pat Convery, a moderate Irish nationalist.

Masked men in Catholic west Belfast hijacked a bus, ordered it to be abandoned outside a police station, and claimed a bomb had been hidden on the top deck of the bus. British Army explosives experts later declared the threat a hoax.

In the town of Lurgan, southwest of Belfast, masked youths in a Catholic district called Kilwilkie threw Molotov cocktails both at police and at a passenger train stopped in the town. The engineer drove the train away after Molotov cocktails hit the side of one cabin, but it didn't catch fire and none of the 55 passengers on board was reported hurt.

In the nearby town of Armagh, several hundred Irish nationalists in another Catholic district gathered around the burning hulk of a hijacked vehicle. Police monitored that scene but didn't intervene.

Each July, Northern Ireland's traditional Orange Order parades raise sectarian passions to boiling point. But British restrictions imposed on Orange routes since 1998 have largely stopped the Protestants — accompanied by "kick the pope" bands of tattooed men playing fife and drum — from parading past most Catholic districts.

Still, authorities have failed to negotiate alternative routes for some parades, including the one past Ardoyne's row of shops on Crumlin Road. The thoroughfare connects one Orange lodge to central Belfast.

The disputed Ardoyne parade involves a single Orange lodge of about 30 men and an accompanying band of about 50 men and boys. But it attracts several hundred Protestant supporters to match the Catholic crowds opposed to it, with police caught in the middle each summer.

Monday's parade passed the conflict zone after a two-hour delay. Some marchers in suits and ties shielded themselves with umbrellas as they walked quickly on a roadway littered with stones and broken glass. No marchers appeared to be hit.

The Orange Order commemorates July 12 — also known as the Glorious Twelfth, an official holiday in Northern Ireland — as the date when their community, descended largely from 17th-century Scottish settlers, secured their place in northeast Ireland versus Catholic natives.

On July 12, 1690, the forces of Protestant King William of Orange defeated the army of his dethroned Catholic rival, James II, at the Battle of the Boyne south of Belfast.

Earlier Monday, at 18 Orange rallying points across Northern Ireland, tens of thousands of Protestants enjoyed impromptu picnics in farm fields as their leaders read religious and political proclamations over loudspeakers. They asserted that William's 1690 victory "established civil and religious liberty," while Northern Ireland's political union with Britain remains "a heritage worthy of being passed down untarnished to future generations."

The Orange Order provides a grass-roots umbrella for Protestants from more than 50 denominations and sects. The order forbids its members to marry Catholics or attend Catholic services. It was a driving force behind the establishment of Northern Ireland as a new part of the United Kingdom in 1921, when the predominantly Catholic rest of Ireland won independence from Britain.